1. Field of the Invention
The present invention relates to a light control element which includes optical waveguides provided in substrates having electro-optical effects and which act as a light modulator or an optical switch.
2. Description of the Related Art
It is difficult to modulate two or more planes of polarization with a conventional light control element including a single-crystal, such as a LiNbO3 (lithium niobate) single-crystal, having an electro-optical effect because of the anisotropy of LiNbO3. In order to solve this problem, for example, Japanese Patent 2615022 discloses a light control element including a plurality of optical waveguides which are arranged in the same plane and which have different functions. End surfaces of the optical waveguides are in contact with each other, so that light beams traveling through the optical waveguides are combined.
Japanese Unexamined Patent Application Publication No. 7-56199 discloses a waveguide-type optical switch that is a polarization-independent optical switch independent of the polarization of incident light.
The light control element disclosed in Japanese Patent 2615022 will now be described with reference to FIG. 1.
FIG. 1 is a perspective view of the light control element disclosed in Japanese Patent 2615022. The light control element is a light modulator that converts linearly polarized incident light into arbitrarily polarized light (elliptically polarized light). Two optical waveguides 3 and 4 are in contact with each other at crystal end surfaces 13 and 14 of optical waveguide substrates 1 and 2, respectively, and are optically combined with each other. The substrates 1 and 2 are made of single-crystalline LiNbO3 and include the optical waveguides 3 and 4, respectively, formed by thermally diffusing titanium ions into surface portions of the substrates 1 and 2. The optical waveguides 3 and 4 have a slightly large refractive index. The orientations 11 and 12 of the substrates 1 and 2, respectively, form an angle of 45 degrees. Electrodes 5 and 6 and electrodes 7 and 8 are arranged at both sides of waveguide 3 and 4, respectively, and are supplied with predetermined voltages from power supplies 9 and 10, respectively, whereby light traveling through the optical waveguides 3 and 4 is modulated. Incoming light 17 entering the optical waveguide 3 through an end surface 15 of the substrate 1 is modulated differently in the optical waveguides 3 and 4 and then emitted from an end surface 16 of the substrate 2.
FIG. 2 is a plan view of an optical waveguide, disclosed in Japanese Unexamined Patent Application Publication No. 7-56199, acting as a waveguide-type polarization-independent optical switch. This optical waveguide includes a LiNbO3 substrate 21, Ti-diffused LiNbO3 optical waveguides 22, a switching positive electrode 23, a switching negative electrode 24, a thin-film wave plate insertion groove 25, a thin-film half-wave plate 26, and bonding wires 27 for connecting electrodes.
The wave plate is placed halfway through the waveguides and polarization is rotated by 90 degrees, thereby achieving an optical switch operating independently of polarization.
For the optical waveguide disclosed in Japanese Patent 2615022, a common configuration for connecting the substrates having a plurality of different functions is disclosed; however, no detailed configuration for enabling a low-voltage operation independently of polarization is disclosed.
In a configuration disclosed in Japanese Unexamined Patent Application Publication No. 7-56199, the ½-wave plate needs to be positioned halfway through the waveguides, which are light paths, and therefore there is a problem in that junctions therebetween have a large optical loss. Furthermore, there is a problem in that the extinction ratio is wavelength-dependent, because the rotation of polarization by the ½-wave plate is wavelength-dependent.